P0300 Random Misfire Diagnosis and Fixes for 2003 Volkswagen Passat 1.8T

I own a 2003 Volkswagen Passat 1.8T with over 300,000 miles. The car has been running smoothly with only a slight misfire until recently. After the engine light came on, I realized it was triggered by a P0300 code indicating random cylinder misfires (cylinders 1 to 4). Since I needed an emissions test for my license plate renewal, I tried multiple fixes to clear the code. I initially used injector cleaner in gasoline several times to clean fuel injectors—no improvement. I also replaced the gas filter and installed new spark plugs. However, this led to a new P0171 code (rich mixture). I then replaced both the MAF sensor and two oxygen sensors (front and rear), suspecting a vacuum leak due to brittle plastic vacuum hoses. I replaced several hose fittings. I later discovered that my crankcase breather tube was broken, so I installed a new intake gasket as a fix. Despite these repairs, the car now runs worse than before—idle is rough, and the engine light blinks at idle. The P0300 code remains unresolved. I suspect ongoing issues with fuel delivery or ignition components, but I'm unsure whether the root cause lies in the engine management system, fuel injectors, or a deeper mechanical fault. I’m seeking advice on how to properly diagnose and fix this persistent random misfire issue without further damaging the vehicle or increasing repair costs.